The disclosure generally relates to interconnects for use with solid oxide fuel cells (“SOFCs”), and which may be used in planar solid oxide fuel cells (“PSOFCs”), and ferritic stainless steels that may be used to form interconnects for SOFCs. For example, certain non-limiting embodiments disclosed herein relate to interconnects that comprise at least one via that when subjected to an oxidizing atmosphere at an elevated temperature develops a scale comprising a manganese-chromate spinel on at least a portion of a surface thereof, and at least one gas flow channel that when subjected to an oxidizing atmosphere at an elevated temperature develops an aluminum-rich oxide scale on at least a portion of a surface thereof. Other non-limiting embodiments relate to interconnects for SOFCs comprising ferritic stainless steel and a metallic material that resists oxidation under certain operating conditions of the SOFCs. Methods of making interconnects for SOFCs and PSOFCs comprising the disclosed interconnects are also described.
Solid oxide fuel cells are fuel cells that are constructed entirely of solid-state materials. Typically SOFCs use a fast oxygen ion-conducting ceramic (typically yttria-stabilized zirconia or “YSZ”) as the electrolyte, and operate in a temperature range of about 500° C. (932° F.) to 1000° C. (1832° F.) to facilitate solid-state transport. A single SOFC “cell” or subunit includes an anode and a cathode separated by the solid electrolyte. Because current generation SOFCs typically operate at temperatures up to about 1000° C., the anodes and cathodes are generally constructed from ceramic materials to avoid environmental degradation. Both the anode and cathode layers contain a network of interconnected pores though which gases may pass and are good electrical conductors (e.g., they exhibit essentially no ionic conductivity). In current generation SOFCs, the anode is typically formed from an electrically conductive nickel/YSZ composite (a ceramic/metal composite or “cermet”), wherein the nickel provides a continuous electrically conductive path, while the YSZ serves to reduce the coefficient of thermal expansion of the overall composite and to prevent the network of pores from sintering shut. The cathode may be based on, for example, lanthanum manganate (LaMnO3), typically doped with strontium (replacing some of the lanthanum to yield La1-xSrxMnO3) to improve its electrical conductivity. The electrolyte is typically a thin (relative to the anode and cathode) layer of fully dense YSZ.
During operation of the SOFC cell, an oxidant (such as O2 or air) is fed into the fuel cell near the cathode of the cell, where it accepts electrons from an external circuit in the following half-cell reaction:½O2(g)+2e−=O−2 
Oxygen ions generated in the half-cell reaction at the cathode pass through the YSZ electrolyte by solid-state diffusion to the electrolyte/anode interface, where they can react with a fuel, such as hydrogen, that has been introduced to the SOFC near the anode. Operationally, pure hydrogen can be used as supplied, while a hydrocarbon fuel such as methane, kerosene, or gasoline generally must be partially combusted, or reformed, to hydrogen and carbon monoxide. This may be accomplished within the fuel cell, aided by the high operating temperature and by steam injection. The fuel gas mixture penetrates the porous anode to the anode/electrolyte interface, where it reacts with the oxygen ions from the YSZ electrolyte in the following half-cell reaction:H2(g)+O−2=2e−+H2O(g) 
As indicated above, this half-cell reaction releases electrons that re-enter the external circuit. To maintain overall electrical charge balance, the flow of electrical charge due to oxygen ion transport through the electrolyte from cathode to anode is balanced by the flow of electrical charge due to electron conduction in the external circuit. The flow of electrons in the external circuit provides an electrical potential of approximately one volt. To generate larger voltages, fuel cells are typically not operated as single units but, instead, as “stacks” composed of a series arrangement of several individual cells with an “interconnect” joining and conducting current between the anode and cathode of immediately adjacent cells. A common stack design is the flat-plate or “planar” SOFC (or “PSOFC”). In a PSOFC, at least two, and preferably more, SOFCs are stacked together in a repeating sequence, wherein each individual SOFC is separated by an interconnect positioned between the anode of one SOFC and the cathode of an immediately adjacent SOFC within the stack.
Depending upon the design of a PSOFC, the interconnect can serve several functions, including separating and containing the reactant gases and providing a low resistance path for current so as to electrically connect the cells in series. An interconnect may also be termed a “bipolar plate” or a “separator” depending upon its function(s) in the fuel cell. In general, the interconnect material must withstand the harsh, high-temperature conditions within the cells; must be suitably electrically conductive (including any oxides or other surface films that form on the material); and must have a coefficient of thermal expansion (CTE) that is sufficiently similar to the CTE of the ceramic electrodes within the cells to ensure the requisite structural integrity and gas-tightness of the fuel cell stack.
Initial PSOFC designs used a doped lanthanum chromate (LaCrO3) ceramic material as the interconnect material. LaCrO3 ceramic does not degrade at the high temperatures at which SOFCs operate and has a coefficient of thermal expansion that substantially matches the other ceramic components of the fuel cell. LaCrO3 ceramic, however, is brittle, difficult to fabricate, and extremely expensive. To address these deficiencies, metallic interconnects have been proposed for use in PSOFCs. These include interconnects formed from nickel-base alloys, such as AL 600™ alloy, and certain austenitic stainless steels, such as the 300 series family, the prototype of which is Type 304 alloy. Ferritic stainless steels, such as ALFA-II™ alloy, E-BRITE® alloy and AL 453™ alloy also have been proposed for use in PSOFC interconnects. Table I provides nominal compositions for the foregoing nickel-base and stainless steel alloys, all of which are available from Allegheny Ludlum Corporation, Pittsburgh, Pa.
TABLE IComposition (weight percent)AlloyNiCrFeAlSiMnOtherAL 453 ™0.3 max.22bal.0.60.30.30.06Ce + Laalloymax.E-BRITE ®0.15 max. 26bal.0.10.20.051MoalloyALFA-II ™0.3 max.13bal.30.30.40.4TialloyAL 600 ™Ni + Co15.58—0.20.25—alloybal.Type 304818bal.————alloy
At the operating temperatures typical for current generation SOFC, the partial pressure of oxygen (or “pO2”) near the anode of a SOFC is generally lower than the pO2 required for various metals commonly used as electrical conductors (e.g., copper and nickel) to form oxides. However, the pO2 near the cathode of a SOFC is generally higher than the pO2 required for oxide formation. Accordingly, there is a tendency for surface oxide layers to form on interconnects made from these metals when exposed to the oxidant proximate the cathode of the SOFC.
Since metals generally form oxides that either have a high electrical resistivity at the temperatures typical of PSOFC operation or rapidly thicken with time, the area specific resistance (or “ASR”) of metal interconnects, as well as the resistivity of the PSOFC stack into which they are incorporated, tends to increase with time during operation of the PSOFC. For example, certain alloys, upon exposure to oxygen at high temperatures, form surface oxides that either thicken at an extremely slow rate (for example, the Al2O3 scale of ALFA-II® alloy) or are highly electrically conductive (for example, the NiO scale of pure or dispersion-strengthened nickel). However, the underlying mechanism that controls these two seemingly disparate factors (resistivity and rate of oxide formation) is essentially the same (the electronic defect structure of the oxide). Consequently, there are very few metal oxides that are both slow growing and electrically conductive.
Stainless steels have attracted interest as potential interconnect materials, in part, because in their conventional form they develop a scale consisting primarily of chromium oxide (Cr2O3). This oxide scale is both relatively slow growing and reasonably electrically conductive at typical, current generation SOFC operating temperatures. Ferritic stainless steels in particular have certain properties that make them attractive for PSOFC interconnect applications, including low cost, good fabricability, and CTE compatible with ceramic. Nevertheless, the oxidation of stainless steel interconnects during operation of a PSOFC may lead to an undesirable degradation of electrical properties of the PSOFC over time.
Another potential drawback to the utilization of stainless steels in PSOFC applications is “poisoning” of the porous electrodes, and in particular the cathodes, used in the SOFCs by chromium-bearing vapor species that may evolve from the chromium-rich oxide scale on the surface of the stainless steel during operation, particularly in the presence of water or hydrogen. Because water vapor is often present in the gas streams of an operational PSOFC, the formation of volatile chromium oxy-hydroxides (e.g., CrO2(OH)2) at lower temperatures can exacerbate the problem. Additionally, solid state diffusion of chromium from the interconnect to the adjoining cathode may occur during operation of the PSOFC and may also contribute to cathode poisoning. While the formation of a manganese-chromate spinel layer on the surface of a stainless steel interconnect may reduce such chromium migration (e.g., the evolution of chromium-bearing vapor species and/or solid state diffusion of chromium) during operation of a PSOFC, if sufficient chromium is present at the surface of the interconnect, chromium migration leading to cathode poisoning may still occur.
Various structures have been proposed for SOFC interconnects. For example, U.S. Pat. No. 6,326,096 discloses an interconnect for solid oxide fuel cells having a superalloy metallic layer with a anode-facing face, a cathode-facing face, and a metal layer, preferably nickel or copper, on the anode-facing face (see Abstract). Disclosed superalloys include Inconel® alloys, Haynes® alloys, Hastelloy® alloys, and austenitic stainless steels (see col. 4, lines 60-63).
U.S. Pat. No. 4,781,996 discloses a separator plate that is laminated on the back surface of each of the anode and the cathode of a fuel cell, and is made of an nickel-containing iron alloy containing from about 25-60% nickel in order to match the linear expansion coefficient of the separator plate with the expansion coefficient of the electrolyte plate (see col. 3, lines 18-27). Further, an oxidation resistant treating material is bonded to the cathode side of the separator and an alkali corrosion-resistant treating material is bonded to the anode side of the separator (see col. 4, lines 24-29).
U.S. Pat. No. 5,227,256 discloses a bimetallic separator plate for a fuel cell in which stainless steel may be used on the cathode face and nickel or copper on the anode face (see col. 11, lines 34-38). Further, the nickel or copper may be about 10 percent of the thickness of the separator plate (see col. 1, lines 38-40). Specifically disclosed are 300 series stainless steel alloys (see col. 11, lines 40-42). U.S. Pat. No. 5,733,683 discloses a metallic bipolar plate for high-temperature fuel cells, the plate having a body having surfaces adapted to contact electrodes of the fuel cells and passages having walls confining gases. The body of the plate is composed of a chromium-containing alloy oxidizable at the surface to form chromium oxide, the alloy being enriched with aluminum at least in regions of the walls in direct contact with the gases (see Abstract). Aluminum enrichment can be carried out using a conventional aluminum diffusion process, wherein the plate is coated with a powder mixture of inert material (such as Al2O3), a chloride/fluoride activator (such as NaCl) and aluminum powder, and exposed at 600° C. to 1300° C. under argon, or coated using CVD or PVD (see col. 3, lines 43-57). Thereafter, surfaces of the plate wherein aluminum enrichment is not desired (for example the electrical contact surfaces) are ground to remove the enriched layer of material. In order to accommodate grinding, the body of the plate is over-sized to account for material removal (see col. 3, lines 57-62).
Canadian Patent No. 2,240,270 discloses a bipolar plate consisting of a chromium oxide-forming alloy with an electrically insulating, corrosion reducing layer in the region of the gas guiding surfaces and cobalt, nickel or iron enrichment layers in the region of the electrode contact surfaces (see Abstract). As discussed above with respect to U.S. Pat. No. 5,733,683, grinding is required to remove the electrically insulating layer from the electrode contact surfaces; accordingly, the plate is over-sized to account for the material removal (see page 8, lines 10-15).
U.S. Patent Publication 2003/0059335 discloses a high temperature material that consists of a chromium oxide forming iron alloy including 12 to 28 wt % chromium, 0.01 to 0.4 wt % La, 0.2 to 1.0 wt % Mn, 0.1 to 0.4 wt % Ti, less than 0.2 wt % Si, and less than 0.2 wt % Al, wherein at temperatures of 700° C.-950° C. the material forms a MnCr2O4 spinel phase and which can be used to form a bipolar plate for a SOFC (see Abstract and paragraph [0032]).
There remains a need for interconnects for SOFCs that have oxidation properties that are tailored to the environmental conditions experienced by interconnects during operation of a PSOFC, that do not require high-temperature treatments, over-sizing or the use of expensive superalloys to achieve desired properties, and that may provide for improved electrical performance of the PSOFCs into which they are incorporated. Further, there is a need for ferritic stainless steels that are compositionally tailored for the SOFC environment and from which such interconnects may be fabricated.